Printed Circuit Board

ABSTRACT

A printed circuit board that includes conductive layers separated by insulation layers of dielectric material, at least one conductive layer being patterned and having at least one signal line embedded in an insulation material, whereby a conductive ground plan layer, separated by the insulation material and lying in a predetermined distance (d) from the at least one signal line includes a ground plane area associated to and extending along the at least one signal line, the conductive layer associated to and extending along the at least one signal line is provided with openings therein. Preferably the openings are spaces between conducting stripes, extending, seen from above, across the at least one signal line, the conducting stripes being integrally connected with the conductive remainder of the conductive layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a printed circuit board (PCB) and in particular to a printed circuit board comprising conductive layers separated by insulation layers of dielectric material, at least one conductive layer being patterned and having at least one signal line embedded in an insulation material, whereby a conductive layer, separated by the insulation material and lying in a predetermined distance from the at least one signal line includes a ground plane area associated to and extending along the at least one signal line.

2. Description of the Related Art

Increasing miniaturization and extreme electronic component density as well as the necessity to transfer large amounts of data at high speed, e.g. at rates of 1 to 5 Gbps, can create serious problems with respect to signal integrity in PCBs. A specific problem here is the desire to configure signal lines with predetermined high impedance. In order to avoid signal losses due to reflexions at interfaces with other signal lines, it is necessary to adjust the line impedance already during the manufacture of a PCB as accurately as possible. Leakage currents, which should be as small as possible, constitute a further problem. A further problem, typically for high speed lines, is associated with return currents in HF-applications. Diverging return current paths lead to an uncontrolled impedance of the transmission line, build by a signal line and a ground plane. Accordingly it is generally recommended to avoid any splitting of the ground plane. Finally the current paths may be considered as an antenna, receiving and transmitting signal energy, thereby creating electromagnetic interference.

FIG. 1 is a cross sectional view of a PCB having three conducting layers, separated by two dielectric insulation layers. In this example of a conventional PCB 1 the bottom layer 2 is a structured layer of conductive material, mostly copper, having two signal lines 3, 4. Separated by a dielectric layer 5 there is arranged a further conductive layer 6, acting as a ground plane. This layer may be structured too, however in “electrically” proximity of the signal lines this layer 6 is made continuous. The conductive layer is followed by another dielectric insulation layer 7 and here the uppermost layer is a further conductive layer 8, which may be structured in a well-known manner. Electrical field lines are indicated schematically by broken lines.

The impedance of the signal transmission line is a function, amongst others, of the distance between the signal lines 3, 4 and ground, defined primarily by the conductive layer 6, the width of the lines 3, 4 and the relative permittivity ε_(r) of dielectric layer 5. At a given width of a signal line higher impedance may be reached by using a dielectric layer with a low relative permittivity ε_(r) and/or by increasing the distance between the signal line and the conductive layer 6. Since the relative permittivity is determined in most cases by the commercially available materials, mostly prepregs, FR4, Polyimide etc., one have to increase the distance between the signal lines 3, 4 and the conductive layer 6, which leads to an undesirable increase of the thickness of the PCB as such. The current standard impedance requirement of 90-100 ohms is almost impossible to achieve for a strip line with a single dielectric layer multilayer microvia stack up of a HDI (High Density Interconnect) PCB. This puts pressure on the designers, who in some cases need to introduce an additional layer just to reach the required impedance in some specific tight areas of the stack up. It is further to mention that in order to reduce loss of electrical signal in high-frequency applications the PCB must show low dielectric constant and low dielectric loss.

SUMMARY

An object of the present invention is to provide a PCB with signal lines, having predefined impedance, which can be adjusted already during the manufacture of the PCB.

A further aspect of the present invention is to provide a PCB with signal lines with improved signal integrity, e.g. well defined return current paths.

Quite another aspect of the present invention is to provide a PCB with reduced problems caused by the creation of electromagnetic interference.

Another object of the invention is the provision of a PCB with signal lines having, despite of small thickness, reduced loss in high-frequency regions.

Thus the present invention provides a printed circuit board comprising conductive layers separated by insulation layers of dielectric material, at least one conductive layer being patterned and having at least one signal line embedded in an insulation material, whereby a conductive ground plane layer, separated by the insulation material and lying in a predetermined distance from the at least one signal line includes a ground plane area associated to and extending along the at least one signal line, the conductive layer associated to and extending along the at least one signal line is provided with openings therein.

Preferably the openings are spaces between conducting stripes, extending, seen from above, across the at least one signal line, the conducting stripes being integrally connected with the conductive remainder of the conductive layer.

The conducting stripes may cross the signal line at an angle of 30°-60°, preferable at an angle of 45°.

The width of the conducting stripes can be smaller than the distance between adjacent stripes.

The ratio of the width of the conducting stripes and the distance between adjacent stripes is preferably between 0.1 and 0.8.

A preferred embodiment of the invention comprises at least a support core, a first structured conductive layer including the at least one signal line arranged on the support core, followed by a first prepreg-layer, a second structured conductive layer including the ground plane area layer with openings therein, followed by a second prepreg-layer and a third conductive layer arranged on said second prepreg layer.

A printed circuit board according to the invention may further comprise two differential signal lines, arranged in parallel, the ground plane area being associated to both signal lines.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a PCB having two signal lines according to prior art.

FIG. 2 is a schematic cross-sectional view of a PCB according to the invention having two signal lines and a hatched ground plane.

FIG. 3 is a simplified and schematic plane view showing the arrangement of a ground plane area with respect to a single signal line.

DETAILED DESCRIPTION

Embodiments of a PCB according to the invention will be described below in more detail with reference to the accompanying drawings. For same or similar components same reference numerals are used in order to avoid redundant explanations.

A printed circuit board 9 according to the invention, as shown in FIG. 2 comprises a bottom layer, namely a support core layer 10, made of a dielectric material, as for instance a FR 4 material or polyimide. Next layer is a conductive layer 11, built on the support core layer 10, the conductive layer being patterned and having in the present example two signal lines 12, 13, extending in a direction perpendicular to the plane of FIG. 2. The signal lines 12, 13, for example, may serve as differential signal lines, preferable with an usual impedance of 50-100 ohm. The support core layer 10 with the structured conductive layer 11 is followed by a first prepreg-layer 14, which covers the conductive layer 11. In this way the signal lines 12, 13 are completely embedded in dielectric material, i.e. on their lower surface by the support core layer 10 and on their upper surface and their sides by the first prepreg-layer 14.

Next follows a second structured conductive layer 15 covered by a second prepreg- layer 16. Conductive layer 15 comprises an area acting as a ground plane area 17 associated to and extending along the signal lines 12, 13. This ground plane area 17 of conductive layer 15 is provided with openings 18. In the example shown these openings are spaces between conducting stripes 19, extending, seen from above, across signal line 12, 13, the conducting stripes being integrally connected with the conductive remainder 20 of conductive layer 15.

This can be seen better in FIG. 3, which is a plane view showing the arrangement of the ground plane area 17 of conductive layer 15, openings 18 and the stripes 19, arranged above one single signal line 12.

Returning to FIG. 2 the stripes 19 are completely embedded in dielectric material, i.e. on their lower surface by the first prepreg-layer 14 and on their upper surface and their sides by the second prepreg-layer 16, filling the openings 18 too.

Top layer of the circuit board 9 is a third conductive layer 21 which may be patterned, having conductor paths, not shown in the drawings. At all it is to remark that FIG. 2 only illustrates a part of a PCB with the features essential for the invention. In general PCB 9 will have larger size and will not be limited to a certain number of layers. Furthermore PCB will have vias or microvias interconnecting conductor paths of different layers. Here too electrical field lines are indicated schematically by broken lines and it should be mentioned that parts of the conductive layer 11, adjacent to signal lines 12, 13 act partly too as ground plane.

It has been found that a very stable performance of the strip line can be achieved if the conducting stripes cross the signal line at an angle α of 30°-60°, preferably at an angle α of 45°. In this way it is possible to raise the impedance without creating severe problems with regard to return currents and attenuation.

It is recommendable that the width w of the conducting stripes 19 is smaller than the distance s between adjacent stripes. A preferred ratio of the width w of the conducting stripes 19 and the distance s between adjacent stripes is between 0.1 and 0.8. This can be seen in FIG. 3 however it is to emphasize that FIGS. 2 and 3 are only schematic representations of the invention and not drawn to scale.

Support core 10 is made by impregnating reinforcing material like glass fibres with resin—e.g. epoxy resin, available under grade designations such as FR-4, FR-5 or others or by using polyimide resin. First and second prepreg-layers 14, 16 advantageously consist of FR-4, but other dielectric materials, suitable for a lamination process, may be used.

A typical thickness of conductive layers, usually consisting of copper ranges between 1 and 20 μm, a typical thickness of the dielectric layers between 5 and 40 μm.

PCB 9 may also be a flex type PCB or a rigid-flex type PCB and in this cases other materials for the dielectric layers and thinner conducting layers may be used at least for flexible parts of the PCB.

The invention is not restricted to differential signal lines and may include only one line or more than two signal lines.

While the foregoing description is directed to various preferred embodiments of the invention, it should be noted that variations and modifications will be apparent to the skilled person without departing from the scope of the invention, defined by the annexed claims. 

1. A printed circuit board (9) comprising conductive layers (11,15, 21) separated by insulation layers (14, 16) of dielectric material, at least one conductive layer (11, 15) being patterned and having at least one signal line (12, 13) embedded in an insulation material (10, 14, 16), whereby a conductive layer (15), separated by the insulation material and lying in a predetermined distance (d) from the at least one signal line includes a ground plane area (17) associated to and extending along the at least one signal line, characterized in that the conductive layer (15) associated to and extending along the at least one signal line (12, 13) is provided with openings (18) therein.
 2. A printed circuit board (9) according to claim 1, characterized in that the openings (18) are spaces between conducting stripes (19), extending, seen from above, across the at least one signal line (12, 13), the conducting stripes being integrally connected with the conductive remainder (20) of the conductive layer (15).
 3. A printed circuit board (9) according to claim 2, characterized in that the conducting stripes (19) cross the signal line at an anglea of 30°-60°.
 4. A printed circuit board (9) according to claim 3, characterized in that the conducting stripes (19) cross the signal line at an angle α of 45°.
 5. A printed circuit board (9) according to claim 2, characterized in that the width (w) of the conducting stripes (19) is smaller than the distance (d) between adjacent stripes.
 6. A printed circuit board (9) according to claim 5, characterized in that the ratio of the width (w) of the conducting stripes (19) and the distance (d) between adjacent stripes is between 0.1 and 0.8.
 7. A printed circuit board (9) according to claim 1, characterized in that it comprises at least a support core (10), a first structured conductive layer (11) including the at least one signal line (12, 13) arranged on the support core, followed by a first prepreg-layer (14), a second structured conductive layer (15) including the conductive ground plane layer (17) with openings (18) therein, followed by a second prepreg-layer (16) and a third conductive layer (21) arranged on said second prepreg layer.
 8. A printed circuit board (9) according to claim 1, characterized in that it comprises two differential signal lines (12, 13), arranged in parallel, the ground plane area (17) being associated to both signal lines. 